This invention relates to a carrier wafer, a method for holding a flexible substrate by means of a carrier wafer of this type and a method for the manufacture of a carrier wafer of this type. Carrier wafers are used to hold thin and ultra-thin substrates such as semiconductor components, for example.
Thin semiconductor components have been widely used in the microelectronics industry in recent years. Examples of such uses are integrated circuits for chip cards in which the thickness of the silicon components is currently approximately 150 μm and less. Increasingly thinner wafers are also being used in the manufacture of solar cells. Chips with a thickness of 100 μm and less are also used in power semiconductors. Last but not least, electronic components made of organic semiconductors on thin plastic films are also used in polymer electronics, such as organic light-emitting diodes (OLED), for example.
One problem in the handling of such thin substrates is the danger that the substrate may break. Thin substrates of this type also lose their stability and bend under their own weight and inertia, frequently by several millimeters. Therefore normal handling devices that remove the substrate from a kiln and transport it through various processing stations can no longer be used.
An additional problem is presented by the sharp edges of the thin substrates, which cause problems during transport and handling.
Hence, there is a need to develop handling techniques that make it possible to handle and process thin substrates of this type.
It is known that substrates such as thin wafers or chips can be handled by adhesively connecting them to a carrier plate. In this method, it is decisive that the thin substrate must be supported by a thick carrier wafer. The substrate is thereby stabilized and held flat.
The carrier wafer can be connected to the substrate by an adhesive, for example, such as wax, a thermoplastic adhesive or a foil that is adhesive on both sides. In certain situations the use of adhesives or films is not appropriate, e.g. because they would be destroyed by high temperatures or by certain chemicals. Under some conditions, adhesive residue remains on the surface of the substrate after the substrate has been processed and removed from the carrier wafer, which results in the undesirable contamination of the equipment.
In plasma plants, the principle of the electrostatic chuck has long been known. After the wafer to be processed is placed on the electrostatic chuck, a direct current is applied between the anode of the plasma reactor and the back side of the electrostatic chuck. The resulting electrical field between the back side of the wafer and the insulation layer of the chuck holds the utilizable wafer securely and makes possible, among other things, a cooling of the back side of the wafer by the admission of small quantities of helium gas. However, the electrostatic chuck described above is a stationary component of the plasma plant and therefore cannot be transported together with the substrate between different processing stations.
DE 10 2006 055 618 A1 describes a mobile holder for thin substrates on which a flat electrode, insulated on all sides and made of conductive material is located. The conductive floating electrode is completely surrounded by insulating material so that it is called a floating electrode. The insulating material can be applied in the form of one or more dielectric layers. This floating electrode can now be charged, for example, by means of a tunnel window, for example, whereby because it is insulated on all sides, the potential of this floating electrode is initially not defined (floating).
To hold a substrate, e.g. a utilizable wafer, first the utilizable wafer is placed on a carrier wafer with a discharged floating electrode. Then the floating electrode is charged, e.g. to a potential from 10 V to 1,000 V with respect to its environment.
After this charging process with a voltage applied temporarily, for example to a tunnel window in the insulation of the floating electrode, the electrical charges are permanently stored in the floating electrode. The wafer is then securely held by the electrostatic forces of the floating electrode and can be processed, transported and stored.
The insulation on all sides ensures that the charged state of the floating electrode is retained for a long period of time. For this purpose, however, great care must be taken during the manufacture of the carrier wafer to guarantee that is remains insulated and to prevent leakage currents. Even extremely small leaks in the insulation would result directly in the discharge of the floating electrode, so that it would no longer be suitable for the long-term holding of a thin utilizable wafer.
After processing, the utilizable wafer is again removed from the carrier wafer by discharging the floating electrode. This can be accomplished, for example, by the application of an oppositely poled high voltage, an alternating voltage with decreasing amplitude and/or by exposure to UV light. The utilizable wafer can then be removed from the carrier wafer almost without force.
A disadvantage with the carrier wafer known from DE 10 2006 555 618 A1 is that if there is the slightest defect in the insulation layers that insulate the electrode on all sides, a leakage current occurs that results in a discharge of the floating electrode. Because the floating electrode in this carrier wafer has a large surface area, the probability of a defect in the insulation layer is very high, so that these carrier wafers frequently fail.